This invention relates to the production of an ion cluster beam, and, more specifically, to processing of the beam to obtain a focused beam of a selected energy distribution of the clusters.
The deposition of thin films upon substrates is an important manufacturing and research tool in a variety of fields. For example, microelectronic devices are prepared by depositing successive film layers onto a substrate to obtain specific electronic properties of the composite. Photosensitive devices such as vidicons and solar cells are manufactured by depositing films of photosensitive materials onto substrates. Optical properties of lenses are improved by depositing films onto their surfaces. These examples are, of course, only illustrative of the thousands of applications of thin film deposition techniques.
In the highly controlled approach to thin-film deposition that is characteristic of applications wherein a high quality film is required, the film is built up by a successive deposition of monolayers of atoms, each such layer being one atom thick. The mechanics of the deposition process can best be considered in atomistic terms. Generally, in such a process the surface of the substrate must be carefully cleaned, since minor contaminant masses or even contaminant atoms can significantly impede the deposition of the required highly perfect film. The material of the film is then deposited by one of many techniques developed for various applications, such as vapor deposition, sputtering, chemical vapor deposition, or electron beam evaporation.
In another technique for depositing thin films, ionized clusters of atoms are formed in a cluster source. These clusters usually have on the order of about 1000 (and sometimes up to 10,000) atoms per cluster. The clusters are ionized and then accelerated toward the substrate target by an electrical potential that imparts an energy to the cluster equal to the accelerating voltage times the ionization level of the cluster. Upon reaching the surface of the substrate target, the clusters disintegrate at impact. Each atom fragment remaining after disintegration has an energy equal to the total energy of the cluster divided by the number of atoms in the cluster. The cluster prior to disintegration therefore has a relatively high mass and energy, while each atom remaining after disintegration has a relatively low mass and energy. The energy of the atom deposited upon the surface gives it mobility on the surface, so that it can move to imperfections such as kinks or holes that might be present on the surface. Some of the deposited atoms come to rest in the imperfections, thereby removing the imperfections and increasing the perfection and density of the film. Other approaches to using clusters have been developed, and it appears that deposition using cluster beams is a promising commercial film-manufacturing technique.
As with other beam techniques, it is desirable to have the capability to focus the beam to a selected pattern, such as a well-defined spot, and to move the focused beam pattern relative to the surface of a target, to write a pre-selected pattern onto the surface of the target. Although it has been previously possible to deflect an ion cluster beam over the surface of the target, there has been little control over the cross-sectional shape of the beam.
The use of ion cluster beams presents some problems not encountered with other types of beams. Conventional beams such as electron or unclustered ion beams typically have a fairly narrow energy spectrum of the beam and a wide beam divergence. By contrast, the ion cluster beam, due to the nature of its formation and ionization, has a wide energy spectrum but fairly narrow beam divergence. That is, if one graphs the number of clusters in an ion cluster beam having a particular energy as a function of the energy, the spectrum produced is generally much broader than observed for similar types of plots for electron beams or beams of unclustered ions.
Even if the ion cluster beam could be focused to a small cross-sectional beam spot, that spot would include clusters of widely varying energies. Those various energies could have a significant adverse result on the quality of the surface deposit or effect produced with the beam. Moreover, as a practical matter, the variation in the energies of the clusters in the beam would not permit a narrowly focused beam to be produced by presently known techniques.
An important aspect of the development of commercially practical ion cluster beam apparatus and procedures is the need for an approach to producing ion cluster beams having a controllable beam cross section and a beam energy spectrum that can be selected to be within limits for particular applications. Such an approach should be compatible with other aspects of cluster beam technology. The present invention fulfills this need, and further provides related advantages.